A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer).
Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical dimension (typically linewidth) of developed photosensitive resist and/or etched product features. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
As the resolution of lithographic processes increases, ever smaller features will created on substrates, below the resolution of current scatterometers. In order to perform scatterometry at higher resolution one can consider using to use shorter wavelengths of radiation. Wavelengths in the ultraviolet (UV) range may be effective for this in principle. However, optical systems for such wavelengths become particularly complex, and feature sizes continue to shrink beyond the resolution of classical optics. Technology roadmaps point to feature sizes smaller than 20 nm, and even smaller than 10 nm in coming years.
While techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM) exist for accurate imaging of even such small features, they are contact-based methods, too slow and costly to be used as a routine inspection tool in mass-production. There is accordingly a desire for new forms of inspection methods and apparatus, particularly ones suitable for measuring mass-produced metrology targets with feature sizes at the resolution of current and next-generation lithographic processes. Ideally, a new inspection method would operate at high-speed and in a non-contact manner, to perform a role similar to that played by scatterometers used in mass-production today.
Raman spectroscopy is a technique known for measuring material characteristics, based on the phenomenon of inelastic scattering. Briefly, the Raman spectrum includes components at wavelengths shifted from the wavelength of an incident radiation beam. The change in wavelength is not caused by any fluorescence effect, but is caused by an exchange of energy between the scattered photons and the material by which it is scattered.
Typically the exchange of energy comprises coupling between the photons and vibrational energy modes of the material's molecules or lattice structure. In U.S. Pat. No. 7,903, 260 a spectroscopic scatterometer is combined with a Raman spectrometer in order to analyze material properties selectively. That is to say, US'260 teaches that, by measuring the Raman spectrum of a signal which is a first order diffraction signal from a periodic grating structure having product-like features, it can be ensured that the Raman spectrum represents the material characteristics of the product-like features. US'260 does not, however, propose any application to products smaller than the resolution of the scatterometer. Nor does it propose using the Raman spectrum as a means to investigate dimensional characteristics of a structure, as opposed to material characteristics.